Designer Mario Buatta, a.k.a. The Prince of Chintz, has spoken of mixing Benjamin Moore's White and Linen White. 50/50 for his idea of a *perfect* white. Keep in mind you can mix in small quantities to experiment. Don't have to dump two quarts into a bucket just to see what the 50/50 mix will look like.

As far as going from 3 colorants to 6 colorants in the combined mix, there's whole lotta color mixin' conversation to be had about that one. Technically, yeah, you are combining into one mixture the total amount of colorants from each paint color.

After that point, however, is where it gets more complicated.

I'm not sure if you can extrapolate from fine art painting to house painting, but students studying art are told to try to mix the exact color they need from the fewest number of different paints -- and no more than three different ones (plus white). The more you add, the likelier you are to get "mud." Nuanced colors are nice but they can quickly slip downhill into mud if fussed with too much. You can neutralize red with green to get a cooler color that is still warm and if you keep going you get a nice brown grey. But if you add other colors especially from other parts of the color wheel, you will get mud unless you really know what you're doing.